1. Field of the Invention
This invention is directed to wellbore sidetracking methods, wellbore milling methods, and apparatuses useful in such methods.
2. Description of Related Art
In a variety of typical milling operations a section of casing cemented in a cased wellbore is milled out and then a whipstock is positioned adjacent a portion of the milled out section with the top of the whipstock contacting the interior surface of the cement or wellbore which was exposed when the casing section was milled out. Further milling of the casing is done by milling systems which are biased against the casing by contacting a concave surface of the whipstock. A mill used with such a system typically mills a portion of the cement between the exterior surface of the casing and the interior surface of the wellbore and then a portion of the formation through which the wellbore extends as the mill moves further into and/or down the casing.
The present inventors have recognized a problem associated with milling in a wellbore in a hard, tough, abrasive earth formation (e.g., but not limited to the Embar formation). Rather than being deflected into such a formation by the concave of a whipstock, the formation itself may deflect the mill against the whipstock resulting in damage to the whipstock and ineffective or incomplete milling of casing.
In a variety of wellbore sidetracking operations a section of casing is milled out from a cased, cemented wellbore; a deflector, e.g. a whipstock is positioned adjacent the milled section; and then a drill bit is inserted into the wellbore to contact the whipstock to be deflected thereby, and then to drill another bore off from the milled section area. Often the portion of milled casing has an upper edge or stub which is contacted by the drill bit as it begins to drill away from the whipstock. Many drill bits will not cut casing, cut it poorly, and/or are damaged by contact with casing.
In a variety of prior art drilling methods (see, e.g. U.S. Pat. Nos. 3,095,039; 2,699,920; and 1,570,518) a whipstock is set in a wellbore with a bottom portion anchored in the wellbore or in a lower tubular in the wellbore and with a top portion resting against the wellbore. Such positioning of a whipstock creates a variety of problems. The whipstock may not be stable since the formation is not necessarily stable, a top of the whipstock may be unsupported or positioned over a void in the formation. The whipstock resting against the wellbore may not produce accurate positioning and therefore drilling may commence in an undesired direction. Also, when a whipstock abuts a formation there may be more open area in which a mill can "walk off" a concave.